This invention relates to barrier laminates for containment of essential oils, flavors and vitamin C in a liquid and for exclusion of oxygen, and to a container for a liquid, particularly a fruit juice, made from those barrier laminates. It also relates to a method of making a barrier laminate of the above described type.
Glass containers have been used for many years to store and transport juice and milk, but have a number of disadvantages including shipping costs for the empty containers, disposal problems, breakage and weight problems. Similarly, blow molded plastic containers for juices and milk are also available, but the empty containers are comparatively expensive to transport and do not prevent loss of vitamin C from a juice product contained in them. In addition manufacturing costs for such all-plastic containers can be prohibitive since expensive equipment is involved in their manufacture.
The advantages of comparatively low container weight, ease of sealing and opening, ease of disposal and low shipping volume for the empty containers (since the containers can be shipped in a collapsed condition), make containers based on a paperboard substrate the current favored choice for marketing juice products and milk. These cartons are known in the trade as "gable top", containers or cartons.
For example, one such carton, or gable-top milk carton, is described in U.S. Pat. No. 3,120,333. Blanks used to make this carton include a paperboard base, which is extrusion coated with resin on both sides. The resin, which may be polyethylene, provides a moisture barrier and means for heat-sealing the carton.
In a typical carton converting operation, once the resin-blanks are scored and cut, the resin on an outer surface of a glue flap and the resin on an inner surface of a carton panel are heated by direct flame application. The carton panels are then folded over to form a flattened tube, the now molten tacky resin on the heated surfaces are pressed together at a downstream nip to form a liquid-tight seam. The cartons, in a flattened tube form, can then be shipped to users such as juice manufacturers or dairies where they are erected, the bottoms heat sealed, filled and their tops finally sealed.
Although considerations of cost make paperboard containers desirable for containing fruit juices and milk, other factors are also important. These involve the choice of a suitable barrier (laminate) for carton construction for retention of flavor and vitamin content during storage. The effect of diffusion of oxygen into the liquid in the container through the barrier laminate and absorption of essential oils from the liquid into the laminate, remain important considerations in the choice of a suitable laminate.
Laminates containing a metal foil have been used to make a foldable paperboard-based carton for a juice product. These metal foil-containing containers do retain the vitamin content and flavors in the juice for a substantial period of time (around ten weeks), but are expensive compared to containers that do not require a metal foil in the laminate from which they are made. Further, metal foil laminates are prone to develop pin holes seriously affecting their ability to contain liquids.
Thus, considerable effort has been devoted to finding the best layer structure in a barrier laminate.
U.S. Pat. No. 4,513,036 describes a laminate useful in making a paperboard-based carton for juices, which does provide an effective barrier to the migration of essential oils and/or flavors, and does not contain a metal foil. This laminate has a paperboard substrate, a layer of polypropylene polymer coated thereon and a polyolefin layer on the polypropylene polymer layer and is heat sealable.
A further advance in the art of making a juice or milk carton resulted from the introduction of a laminate, which also provided an effective barrier for oxygen and thus helped retain vitamin C in the juice stored in a carton made from it. This laminate, which is described in U.S. Pat. No. 4,777,088, comprises from the outer surface to the inner surface, an outer polyolefin coating that provides the heat seal bond, a paperboard substrate that provides the structure of the carton, a nylon layer coated directly on the paperboard substrate, a layer of modified polyethylene (Bynel E 388) directly overlying and in contact with the nylon layer and an inner polyolefin layer in contact with the modified polyethylene layer. Not only does the nylon barrier layer in this laminate help retain vitamin C, but also the laminate helps retain essential oils and flavor as does the laminate of U.S. Pat. No. 4,513,036.
Other barrier laminates capable of excluding oxygen and preventing loss of oils and/or flavors are described in U.S. Pat. Nos. 4,835,025, 4,701,360, 4,861,526, 4,698,246, 4,806,399, 4,977,004, 4,977,009, 4,950,510 and 4,753,832. In U.S. Pat. Nos. 4,701,360, 4,861,526 and 4,698,246 both sides of a paperboard substrate are first flame treated and a layer of low density polyethylene (LDPE) then applied to the outside surface. To the surface of the paperboard which becomes the inside surface of the carton, first, a layer of low density polyethylene is applied directly to the paperboard. Then a nylon barrier layer is applied to that interior low density polyethylene layer with a bonding tie layer between the nylon and polyethylene. Finally, an innermost skin layer is applied to the nylon with another tie layer to improve the adhesion of the layers and to help in heat sealing. This skin layer is ethylene vinyl alcohol polymer. This process is comparatively complicated and involves a substantial number of layers.